This invention relates to sputtering targets, and more particularly to a method and apparatus for increasing sputtering target lifetime.
Sputter deposition of materials, particularly metals, is one of the most widely used techniques for depositing thin films on semiconductor wafers. xe2x80x9cSputteringxe2x80x9d refers to the process of dislodging atoms from a surface by striking the surface with energetic particles such as ionized gas atoms. Aluminum, or other materials (e.g., aluminum alloys, gold, copper, molybdenum, refractory metals, SiO2, and the like), may be sputter deposited onto the surface of a semiconductor wafer by bombarding a source of deposition material (i.e., a sputtering target) with ions accelerated from an ionized gas, typically referred to as a plasma. Specifically, a semiconductor wafer is placed in a deposition chamber beneath a sputtering target, and the chamber is evacuated to remove water and other contaminants from both the wafer and the deposition chamber. The chamber is then backfilled with an inert gas (e.g., argon) and a gas plasma is generated by exposing the inert gas to a high magnitude electric field which ionizes gas atoms or molecules, forming xe2x80x9cionized particles.xe2x80x9d The ionized particles, typically single ionized atoms of the gas are then guided to, via application of an electric field, and strike the target, dislodging atoms or larger particles of the target material. As a result of collision-induced energy transfer between the ionized gas and target atoms, the sputtered target atoms leave the target and transport to and deposit on the semiconductor wafer (located a distance from the sputtering target), forming a thin film of target material on the wafer.
Because of the large energy exchange between the ionized gas atoms and the sputtering target, the sputtering target must be continuously cooled during sputtering to prevent sagging or even melting thereof. Typically, a sputtering target is cooled by attaching (e.g., by soldering, brazing, or adhesively bonding) the target to a rigid cooling cover plate which serves the dual purpose of cooling the target and providing structural rigidity to the target. A cooling cover plate may, for example, comprise a grooved surface for defining cooling fluid passages when the grooved surface is attached to the sputtering target. Cooling is then achieved by passing a cooling fluid through these cooling fluid passages. (Note that in such a configuration, a fluid tight seal must be maintained between the target and the cooling cover plate to prevent cooling fluid from leaking from the cooling fluid passages and contaminating the sputtering chamber.)
While cooling a sputtering target prevents the target from melting, it also produces a large thermal gradient across the target as one side of the target is cooled (the side in contact with the cooling cover plate) and the other side of the target is heated from collisions with energetic gas particles. This thermal gradient can cause the target to bow as the heated side of the target expands more than the cooled side. A substantial thermally induced strain results across the target which may eventually cause the target to separate from the cooling cover plate (i.e., delaminate) during repeated sputtering operations. When target/cooling cover plate delamination occurs, the fluid tight seal between the target and cooling cover plate is broken and the sputtering chamber may become contaminated with cooling cover plate, adhesive, brazing or soldering material, and/or cooling fluid. Moreover, when the fluid tight seal is broken cooling of the target may become less efficient locally, resulting in localized overheating and catastrophic failure of the target/cooling cover plate assembly.
To prevent target/cooling cover plate delamination caused by target bowing (and the sputtering chamber contamination associated therewith) techniques have been developed for securely attaching sputtering targets to cooling cover plates. For example, U.S. Pat. No. 5,433,835 shows a rigid three piece target assembly (including a cooling cover plate, a target backing plate, and a target) which minimizes target bowing. The use of bolts, screws, nitrile epoxies, diffusion bonding, or brazing are set forth as effective techniques which prevent a target from separating from a rigid cooling cover plate/target backing plate assembly.
While such securing means do prevent target/cooling cover plate delamination, the target is subjected to substantial thermally induced strain during sputtering. Microscopic defects within the target may propagate under the influence of this strain, resulting in eventual target failure (i.e., cracking or breaking) before all target material has been exhausted, thus causing target lifetime to end prematurely.
In either eventuality, whether delamination or cracking, when cooling fluid leaks into the chamber, the chamber must be opened, cleaned, and again degassed before wafer processing may continue. Conventional target assemblies therefore undesirably increase the cost of each processed wafer due to premature failure, and due to the expensive materials and assembly associated with machining the rigid cover plate material and afixing the cover plate to the target.
A need therefore exists for a method and apparatus for cooling a sputtering target without straining the target. That is, a need exists for a method and apparatus which maintains a target/cooling cover plate cooling fluid seal without subjecting the target to excessive strain during sputtering and without requiring expensive manufacturing processes.
The present invention provides a cooling cover plate which is substantially compliant, i.e., sufficiently compliant to allow a sputtering target to bow during sputtering deposition due to the thermal gradient existing across the target when one side of the target is sputtered while the other side of the target is cooled, without inducing strain at the targetxe2x80x94backing plate interface sufficient to cause targetxe2x80x94backing plate delamination. Preferably the cooling cover plate is fabricated from a plastic, a polymer, or a similarly compliant composite material (e.g., polyoxymethylene, acrylonitrile-butadiene-styrene (hereinafter xe2x80x9cABSxe2x80x9d), polycarbonate, a polycarbonate/ABS blend, epoxy fiberglass composites, or any other combination thereof). Such materials are advantageous not only for their compliant properties but also because they are inexpensive and suitable for economical manufacturing processes (e.g., injection molding).
The inventive sputtering target assembly is preferably formed by adhesively bonding a compliant cooling cover plate to a sputtering target. Adhesive bonding is preferred as it uniformly distributes tensile stress along the bonded surface unlike the isolated stress produced by bolting or screwing means which isolate stress at each bolt or screw. To ensure compatibility with typical sputtering process environments, the sputtering target assembly is preferably mechanically stable (e.g., free from significant lessening of tensile strength or tensile modulus) at temperatures up to 150xc2x0 C. and resistant to a maximum cooling fluid temperature of 60xc2x0 C. (particularly at any adhesive bondline). As well, the cooling cover plate material is preferably electrically insulating so as to insulate the electrically charged sputtering target from a magnetic rotation mechanism frequently used in sputtering, and/or to insulate the target from the cooling water connections.
Because the inventive cooling cover plate is substantially compliant, it allows the target to bow during sputtering, thereby relieving strain that would otherwise result if the target were mounted to a rigid cover cooling plate. Defect propagation within the target is thereby reduced and target lifetime enhanced. Furthermore, because the cooling cover plate bows with the target, the driving force for target/cooling cover plate delamination is eliminated so that the probability of catastrophic target failure (i.e., leakage of target/cooling cover plate fluid tight seal) is substantially reduced.
Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings.